This invention relates to a method and apparatus for transmitting frequency shift key (FSK) data in packetized format. More particularly, the invention is directed to a method for using a new Real-time Transport Protocol (RTP) payload type to transport FSK data used for voiceband data transmission outside of the voice RTP stream. This technique allows for data compression and reduces the possibility of packet loss for FSK data carried in-band.
While the invention is particularly directed to the art of data transmission for frequency shift key (FSK) data, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications. For example, the invention may be used where it is desirable and practical to packetize any form of data within an RTP payload.
By way of background, general requirements for voiceband data transmission can be found in GR-30-CORE, LSSGR: Voiceband Data Transmission Interface (FSD 05-01-0100), Issue 2 (December 1998), which is incorporated herein by reference. These requirements indicate how to set up and transmit voiceband data using continuous-phase binary frequency-shift-keying (BFSK or, more commonly, FSK). FSK is typically used to deliver caller identification (caller ID) information to analog line customer premises equipment (CPE) during normal ringing (on-hook transmission) and call waiting alerting scenarios (off-hook transmission). It is also used to deliver visual message waiting indication during suppressed ringing (on-hook transmission) scenarios. While these applications are not necessarily all-inclusive, they are a representative sampling of the most commonly used features implementing FSK data.
The FSK transmission rate is 1200 baud. A one (1) bit is represented by a 1200 Hz tone. A zero (0) bit is represented by a 2200 Hz tone.
In general, FSK voiceband data transmission is framed loosely by other voiceband signaling events. For example, caller ID delivery during normal ringing places the FSK transmission a minimum of 500 milliseconds into the silent interval between the first and second cycles of ringing. FSK transmission must complete a minimum of 300 milliseconds prior to the beginning of the second ringing on cycle. For caller ID delivery during call waiting alerting, FSK transmission must start between 50 and 500 milliseconds after completion of the CPE alerting signal (CAS) and the CAS acknowledgement sequence of voiceband tones.
For on-hook transmission, there is a preamble sequence of 300 bits “010101 . . . ” followed by 180 mark bits (ones) and then the data. For off-hook FSK transmission there is an initial marker of 80 mark bits (ones) and then the data. There is no preamble sequence for off-hook transmissions.
GR-30-CORE imposes three generalized message formats for the data: Single Data Message Format (SDMF), Multiple Data Message Format (MDMF), and Generic Data Message Format (GDMF). For SDMF and MDMF, each byte of data is preceded by a space bit (zero) and followed by a zero to 10 mark bits (ones). Since there is typically only one mark bit per byte of data there are normally 10 bits per byte of transmitted data. Then there is an eight-bit checksum, which is followed by zero to 10 mark bits. For GDMF there is no structure or framing (i.e., space and mark bits) to the data and there is no checksum, but there are zero to 10 mark bits at the end of the data.
As such, there is variability to the FSK transmission itself. The transmission, thus, may have different states. In this regard, the FSK transmission may have a preamble only for on-hook transmission, different marker lengths, multiple message data formats, a variable number of mark bits between data bytes and, after the data for SDMF and MDMF, no checksum for GDMF. Additionally, FSK transmissions are somewhat delay sensitive in that they are framed by other signaling events.
As shown in FIG. 1, a network 10 includes a public-switched telephone network (PSTN) as having a circuit switch 14. Also shown is a digital loop carrier (DLC) 16 connected to the PSTN 12 and a residence or user site 18. As shown, the residence 18 includes a caller ID box 20 and a phone 22.
In this system, FSK data is typically transmitted in-line to the residence 18 from the PSTN 12 through the DLC 16 using conventional techniques. These conventional techniques result in the difficulties noted above, e.g., state difficulties and transmission delays. Moreover, the call including FSK data cannot be routed through an IP network before being transmitted to the residence because the FSK data would be lost as a result of compression techniques typically implemented in such networks, or because of the potential for packet loss in these networks.
The present invention contemplates a new and improved data transmission technique that resolves the above-referenced difficulties and others.